Ceramide is present in a stratum corneum of the skin and constructs a lipid barrier necessary for retaining water, and thereby it plays an important role for maintaining moisture. Ceramide in a stratum corneum is produced by degradation of cerebroside with an enzyme called cerebrosidase. It is known that a part of ceramide is changed into phytosphingosine and sphingosine with an enzyme called ceramidase, and they are important as an agent of regulating proliferation and differentiation of cells. In a human skin, seven kinds of ceramides are present, and have different functions, respectively.
However, since ceramide is a substance having high crystallizability, has low solubility in other oil solution, and precipitates a crystal at a low temperature, it was difficult to maintain stability when incorporated into cosmetics. Further, an aqueous ceramide dispersion may be dispersed using surfactants, but it is difficult to make a particle diameter of an dispersion sufficiently small, and thereby the dispersion inferior in transparency may be produced in some cases.
An emulsified composition that contains a specific group of sphingoglycolipid having moisturing effect, preventing effect on chapped skin, and emulsifying effect is disclosed as a composition containing ceramide (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2000-51676).
A cosmetic additive combined with ceramide which contains cholesterol, fatty acid, and water-soluble polymer (for example, refer to JP-A No. H07-187987) is disclosed. As a composition for external use which is excellent in stability under rapidly changing temperature conditions and has good after-use feel, a water-in-oil type emulsified composition obtained by using sphingosines salt formed with a specific fatty acid as an emulsifying agent and adding an oil-soluble antioxidant at a specific ratio (for example, refer to JP-A No. 2006-335692) is disclosed.
As pharmaceutical preparation technique, a method for producing an additive agent for cosmetic in which a crude dispersion solution of sphingoglycolipid is microparticulated using a specified jet flow in order to sufficiently exhibit the emollient effect of sphingoglycolipid is disclosed (for example, refer to JP-A No. 11-310512).
On the other hand, a process for blending specific fatty acids and specific surfactants is disclosed as a technique for transparently solubilizing and stably blending ceramide (for example, refer to JP-A Nos. 2001-139796 and 2001-316217). However, in order for the ceramide to be made transparent and solubilized, it is necessary to increase the amount of surfactant to be added, and therefore, safety and a sense of use may be compromised. On the other hand, if a small amount of surfactant is added in order to obtain a superior sense of use, the ceramide often becomes cloudy or in a translucent milky state and cannot be made transparent and solubilized. In such a case, separation or creaming over time of the ceramide occurs, and it is difficult to secure a sufficient stability over time of the ceramide.
Thus, even by using these technique, it is, at present, not possible to obtain a ceramide dispersion in which the ceramide can be stably dispersed and which has a superior stability over time.